1. Field of the Invention
The present invention relates to a press die, a burring method using the press die, a method of manufacturing a pressed product, and the pressed product.
2. Description of the Related Art
Burring is a press working technique to punch a pilot hole on a workpiece such as a metal sheet, and to form a flange (a boss) along an edge of the pilot hole.
The burring uses a die and a burring punch. The die has a burring hole formed therein, which has a diameter larger than the pilot hole of the workpiece, and substantially the same as an outer diameter of a desired boss. The burring punch in a rod shape has a diameter substantially the same as an inner diameter of the desired boss. The workpiece is then placed on a stage of the die, such that a center of the burring hole of the die and that of the pilot hole of the workpiece are substantially aligned. After that, the burring punch is inserted into the burring hole from a surface of the workpiece on an opposite side of the die. This allows the edge of the pilot hole to be pressed and extended owing to a pressing force of the burring punch, and to be thereby bent along an edge of the burring hole of the die. As a result, a boss is formed on the workpiece (see, for example, Japanese Laid-Open Patent Application, Publication No. 2002-11526).
The burring described above needs to be performed such that a height of the boss and a thickness of the workpiece are uniform in any part, without cracks or fractures which would otherwise be generated upon the burring.
A press die 101 according to a conventional burring method as shown in FIG. 5 needs to form a product having a uniform height of a boss 111 and a uniform thickness of the product. Therefore, according to a rule of thumb, a radial-direction width of a clearance S between a burring hole 103 and a burring punch 104 upon the burring is usually set at about ½ of a thickness of a workpiece 110.
The conventional burring method can obtain the uniform height of the boss 111 and the uniform thickness of the product. However, the conventional burring method has a problem that a large warpage is generated on a planar part 112 surrounding the boss 111. In particular, when the conventional burring method is applied to a relatively thin workpiece 110 (for example, 0.5 mm or less), a notably large warpage is generated on the planar part 112.
The present invention has been made to solve the problem. The present invention proposes: a press die capable of reducing a warpage to be generated on a planar part surrounding a boss, which is formed with burring; a burring method using the press die; a method of manufacturing a pressed product; and the pressed product.